Mapping the cytoskeletal architecture of renal tubules and surrounding peritubular capillaries in the kidney

The human kidney includes ~1 million nephrons which are long U-shaped tubules with convoluted segments that serve as filtration units. During the passage of the ultrafiltrate through a nephron, electrolytes and nutrients are re-absorbed into peritubular capillaries. The fluid remaining in the distal end of the renal tubules flows through the collecting ducts into the ureter. In this study, we generated high-resolution images of mouse kidney sections using confocal microscopy with only two fluorescently tagged biomarkers, F-actin binding phalloidin and CD34 antibodies as a marker for blood vessels. In tile-scan images of entire sections of mouse kidney (composed of >1000 images), the tubule segments are easily identifiable by their F-actin bundles on cell borders and the outlines of the peritubular capillaries by CD34 immunofluorescence. In the inner stripe of the medulla, the vascular bundles composed of vasa recta (straight vessels) could be easily distinguished from the peritubular capillaries by their full circular shapes. The highly vascular inner medulla and the papilla similarly have straight capillaries. About 95% of kidney volume is composed of renal tubules and blood vessels. Thus, our results show that relatively simple cytoskeletal mapping can be used to visualize the structural organization of the kidney. This method can also be applied to examine pathological changes in the kidney.

The Concise Guide to PHARMACOLOGY 2023/24: Ion channels

The Concise Guide to PHARMACOLOGY 2023/24 is the sixth in this series of biennial publications. The Concise Guide provides concise overviews, mostly in tabular format, of the key properties of approximately 1800 drug targets, and over 6000 interactions with about 3900 ligands. There is an emphasis on selective pharmacology (where available), plus links to the open access knowledgebase source of drug targets and their ligands (https://www.guidetopharmacology.org/), which provides more detailed views of target and ligand properties. Although the Concise Guide constitutes almost 500 pages, the material presented is substantially reduced compared to information and links presented on the website. It provides a permanent, citable, point-in-time record that will survive database updates. The full contents of this section can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.16178. Ion channels are one of the six major pharmacological targets into which the Guide is divided, with the others being: G protein-coupled receptors, nuclear hormone receptors, catalytic receptors, enzymes and transporters. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. The landscape format of the Concise Guide is designed to facilitate comparison of related targets from material contemporary to mid-2023, and supersedes data presented in the 2021/22, 2019/20, 2017/18, 2015/16 and 2013/14 Concise Guides and previous Guides to Receptors and Channels. It is produced in close conjunction with the Nomenclature and Standards Committee of the International Union of Basic and Clinical Pharmacology (NC-IUPHAR), therefore, providing official IUPHAR classification and nomenclature for human drug targets, where appropriate.

THE CONCISE GUIDE TO PHARMACOLOGY 2021/22: Ion channels

The Concise Guide to PHARMACOLOGY 2021/22 is the fifth in this series of biennial publications. The Concise Guide provides concise overviews, mostly in tabular format, of the key properties of nearly 1900 human drug targets with an emphasis on selective pharmacology (where available), plus links to the open access knowledgebase source of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. Although the Concise Guide constitutes over 500 pages, the material presented is substantially reduced compared to information and links presented on the website. It provides a permanent, citable, point-in-time record that will survive database updates. The full contents of this section can be found at http://onlinelibrary.wiley.com/doi/bph.15539. Ion channels are one of the six major pharmacological targets into which the Guide is divided, with the others being: G protein-coupled receptors, nuclear hormone receptors, catalytic receptors, enzymes and transporters. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. The landscape format of the Concise Guide is designed to facilitate comparison of related targets from material contemporary to mid-2021, and supersedes data presented in the 2019/20, 2017/18, 2015/16 and 2013/14 Concise Guides and previous Guides to Receptors and Channels. It is produced in close conjunction with the Nomenclature and Standards Committee of the International Union of Basic and Clinical Pharmacology (NC-IUPHAR), therefore, providing official IUPHAR classification and nomenclature for human drug targets, where appropriate.

High-resolution imaging of the actin cytoskeleton and epithelial sodium channel, CFTR, and aquaporin-9 localization in the vas deferens

Vas deferens is a conduit for sperm and fluid from the epididymis to the urethra. The duct is surrounded by a thick smooth muscle layer. To map the actin cytoskeleton of the duct and its epithelium, we reacted sections of the proximal and distal regions with fluorescent phalloidin. Confocal microscopic imaging showed that the cylinder-shaped epithelium of the proximal region has a thick apical border of actin filaments that form microvilli. The epithelium of the distal region is covered with tall stereocilia (13-18 µm) that extend from the apical border into the lumen. In both regions, the lateral and basal cell borders showed a thin lining of actin cytoskeleton. The vas deferens epithelium contains various channels to regulate the fluid composition in the lumen. We mapped the localization of the epithelial sodium channel (ENaC), aquaporin-9 (AQP9), and cystic fibrosis transmembrane conductance regulator (CFTR) in the rat and mouse vas deferens. ENaC and AQP9 immunofluorescence were localized on the luminal surface and stereocilia and also in the basal and smooth muscle layers. CFTR immunofluorescence appeared only on the luminal surface and in smooth muscle layers. The localization of all three channels on the apical surface of the columnar epithelial cells provides clear evidence that these channels are involved concurrently in the regulation of fluid and electrolyte balance in the lumen of the vas deferens. ENaC allows the flow of Na⁺ ions from the lumen into the cytoplasm, and the osmotic gradient generated provides the driving force for the passive flow of water through AQP channels.

Identification and classification of epithelial cells in nephron segments by actin cytoskeleton patterns

The basic functional unit in a kidney is the nephron, which is a long and morphologically segmented tubule. The nephron begins with a cluster of capillaries called glomerulus through which the blood is filtered into the Bowman's space. The filtrate flows through the nephron segments. During this flow, electrolytes and solutes are reabsorbed by channels and transport systems into the capillaries wrapped around the nephron. Many questions related to renal function focus on identifying the sites of expression of these systems. In this study, we mapped whole kidney sections by confocal microscopic imaging of fluorescent phalloidin, which binds to actin filaments. In tile scans (composed of hundreds of images) of these sections, the cortex and the medullary regions (outer and inner stripes of the outer medulla, and inner medulla) could be easily identified by their cytoskeletal patterns. At a higher resolution, we identified distinct features of the actin cytoskeleton in the apical, basal, and lateral borders of the cells. These features could be used to identify segments of a nephron (the proximal tubule, thin and thick segments of Henle's loop, and distal tubule), the collecting duct system, the papillary ducts in the papilla, and the urothelium that covers the pelvis. To verify our findings, we used additional markers, including aquaporin isoforms, cytokeratin 8‐18, and WGA lectin. This study highlights the power of high‐resolution confocal microscopy for identifying specific cell types using the simple probe of F‐actin‐binding phalloidin.

Mapping the sites of localization of epithelial sodium channel (ENaC) and CFTR in segments of the mammalian epididymis

The sperm produced in the seminiferous tubules pass through the rete testis, efferent ducts, and epididymis. The epididymis has three distinct regions known as caput, corpus, and cauda. The transit through the epididymis is an essential process in sperm maturation. The lumen of each epididymal region has a unique fluid composition regulated by many ion channels and transporters in the epithelial cells. The objective of this study was to map the sites of localization of ion channels ENaC and CFTR along the length of the mouse and rat epididymis using confocal microscopic imaging. The integrity of the fine structure of the tissues was verified by fluorescent phalloidin staining of actin filaments visualized by high-resolution confocal microscopy. The 2D and 3D images showed preservation of the stereocilia. Based on these images we determined morphometric parameters of the epithelial cells and ducts. ENaC and CFTR immunofluorescence appeared almost continuously on the apical membrane of caput and in smooth muscle myoid cells. In cauda, CFTR expression was observed continuously in long stretches of epithelium interrupted by clusters of cells that showed no CFTR expression. Similar patterns of localization were observed in both mouse and rat samples. Mutations in the CFTR gene are known to result in male infertility. Based on the widespread presence of ENaC along the epididymis we suggest that mutations in ENaC subunits may also be associated with male infertility. The diverse phenotypes associated with CFTR mutations may be due to malfunction of CFTR at specific subcellular locations in the male reproductive system.

Expression of the epithelial sodium channel (ENaC) in the endometrium - Implications for fertility in a patient with pseudohypoaldosteronism

Pseudohypoaldosteronism type 1 (PHA) is a syndrome of unresponsiveness to aldosterone. The severe form of this disease results from mutations in the genes that encode for the epithelial sodium channel subunits, SCNN1A, SCNN1B, and SCNN1G. A PHA patient under our care failed to conceive after many years and IVF trials. Our earlier studies had shown that ENaC is expressed in the female reproductive tract. We hypothesized that a defective ENaC expression may be responsible for the infertility of the patient. To test this hypothesis we examined ENaC expression in endometrial Pipelle biopsy samples from three healthy women and the PHA patient with an Arg508X mutation in the SCNN1A gene. The formalin fixed samples were reacted with anti-ENaCA (alpha subunit) antisera, followed by secondary antibodies to visualize ENaC expression by immunofluorescence. Confocal microscopy imaging of the samples showed strong ENaC immunofluorescence along the luminal border (apical membrane) of the epithelial cells in Pipelle samples from healthy women. In contrast, none of the samples from the PHA patient showed ENaC immunofluorescence. The Arg508X mutation interrupts the transport of ENaC subunits to the cell surface, yet it would not be expected to disrupt ENaC localization in the cytoplasm. In contrast to endometrium where ENaC is localized in the apical membrane of the epithelial cells, in keratinocytes ENaC is expressed in cytoplasmic pools. Therefore, we examined ENaC immunofluorescence in plucked hair follicles. As expected, ENaC immunofluorescence was detected in the cytoplasm of keratinocytes of both normal and PHA samples. Our results support the hypothesis that lack of expression of ENaC on the endometrial surface may be responsible for the infertility of the PHA patient.

Conservation of the Enzyme-Coenzyme Interfaces in FAD and NADP Binding Adrenodoxin Reductase-A Ubiquitous Enzyme

FAD and NAD(P) together represent an ideal pair for coupled redox reactions in their capacity to accept two electrons and their redox potentials. Enzymes that bind both NAD(P) and FAD represent large superfamilies that fulfill essential roles in numerous metabolic pathways. Adrenodoxin reductase (AdxR) shares Rossmann fold features with some of these superfamilies but remains in a group of its own in the absence of sequence homology. This article documents the phylogenetic distribution of AdxR by examining whole genome databases for Metazoa, Plantae, Fungi, and Protista, and determines the conserved structural features of AdxR. Scanning these databases showed that most organisms have a single gene coding for an AdxR ortholog. The sequence identity between AdxR orthologs is correlated with the phylogenetic distance among metazoan species. The NADP binding site of all AdxR orthologs showed a modified Rossmann fold motif with a GxGxxA consensus instead of the classical GxGxxG at the edge of the first βα-fold. To examine the hypothesis that enzyme-coenzyme interfaces represent the conserved regions of AdxR, the residues interfacing FAD and NADP were identified and compared with multiple-sequence alignment results. Most conserved residues were indeed found at sites that surround the interfacing residues between the enzyme and the two coenzymes. In contrast to protein-protein interaction hot-spots that may appear in isolated patches, in AdxR the conserved regions show strict preservation of the overall structure. This structure maintains the precise positioning of the two coenzymes for optimal electron transfer between NADP and FAD without electron leakage to other acceptors.

Expression of epithelial sodium channel (ENaC) and CFTR in the human epidermis and epidermal appendages

A major function of the skin is the regulation of body temperature by sweat secretions. Sweat glands secrete water and salt, especially NaCl. Excreted water evaporates, cooling the skin surface, and Na⁺ ions are reabsorbed by the epithelial sodium channels (ENaC). Mutations in ENaC subunit genes lead to a severe multi-system (systemic) form of pseudohypoaldosteronism (PHA) type I, characterized by salt loss from aldosterone target organs, including sweat glands in the skin. In this study, we mapped the sites of localization of ENaC in the human skin by confocal microscopy using polyclonal antibodies generated against human αENaC. Our results reveal that ENaC is expressed strongly in all epidermal layers except stratum corneum, and also in the sebaceous glands, eccrine glands, arrector pili smooth muscle cells, and intra-dermal adipocytes. In smooth muscle cells and adipocytes, ENaC is co-localized with F-actin. No expression of ENaC was detected in the dermis. CFTR is strongly expressed in sebaceous glands. In epidermal appendages noted, except the eccrine sweat glands, ENaC is mainly located in the cytoplasm. In the eccrine glands and ducts, ENaC and CFTR are located on the apical side of the membrane. This localization of ENaC is compatible with ENaC's role in salt reabsorption. PHA patients may develop folliculitis, miliaria rubra, and atopic dermatitis-like skin lesions, due to sweat gland duct occlusion and inflammation of eccrine glands as a result of salt accumulation.

Gene Wiki Reviews-Raising the quality and accessibility of information about the human genome

Wikipedia and other openly available resources are increasingly becoming commonly used sources of information not just among the lay public but even in academic circles including undergraduate students and postgraduate trainees. To enhance the quality of the Wikipedia articles, in 2013, we initiated the Gene Wiki Reviews on genes and proteins as a series of invited reviews that stipulated editing the corresponding Wikipedia article(s) that would be also subject to peer-review. Thus, while the review article serves as an authoritative snapshot of the field, the "living article" can continue to evolve with the crowdsourcing model of Wikipedia. After publication of over 50 articles, we surveyed the authors to assess the impact of the project. The author survey results revealed that the Gene Wiki project is achieving its major objectives to increase the involvement of scientists in authoring Wikipedia articles and to enhance the quantity and quality of the information about genes and their protein products. Thus, the dual publication model introduced in the Gene Wiki Reviews series represents a valuable innovation in scientific publishing and biomedical knowledge management. We invite experts on specific genes to contact the editors to take part in this project to enhance the quality and accessibility of information about the human genome.

Epithelial sodium channel (ENaC) family: Phylogeny, structure-function, tissue distribution, and associated inherited diseases

The epithelial sodium channel (ENaC) is composed of three homologous subunits and allows the flow of Na(+) ions across high resistance epithelia, maintaining body salt and water homeostasis. ENaC dependent reabsorption of Na(+) in the kidney tubules regulates extracellular fluid (ECF) volume and blood pressure by modulating osmolarity. In multi-ciliated cells, ENaC is located in cilia and plays an essential role in the regulation of epithelial surface liquid volume necessary for cilial transport of mucus and gametes in the respiratory and reproductive tracts respectively. The subunits that form ENaC (named as alpha, beta, gamma and delta, encoded by genes SCNN1A, SCNN1B, SCNN1G, and SCNN1D) are members of the ENaC/Degenerin superfamily. The earliest appearance of ENaC orthologs is in the genomes of the most ancient vertebrate taxon, Cyclostomata (jawless vertebrates) including lampreys, followed by earliest representatives of Gnathostomata (jawed vertebrates) including cartilaginous sharks. Among Euteleostomi (bony vertebrates), Actinopterygii (ray finned-fishes) branch has lost ENaC genes. Yet, most animals in the Sarcopterygii (lobe-finned fish) branch including Tetrapoda, amphibians and amniotes (lizards, crocodiles, birds, and mammals), have four ENaC paralogs. We compared the sequences of ENaC orthologs from 20 species and established criteria for the identification of ENaC orthologs and paralogs, and their distinction from other members of the ENaC/Degenerin superfamily, especially ASIC family. Differences between ENaCs and ASICs are summarized in view of their physiological functions and tissue distributions. Structural motifs that are conserved throughout vertebrate ENaCs are highlighted. We also present a comparative overview of the genotype-phenotype relationships in inherited diseases associated with ENaC mutations, including multisystem pseudohypoaldosteronism (PHA1B), Liddle syndrome, cystic fibrosis-like disease and essential hypertension.

Proteopedia: Rossmann fold: A beta-alpha-beta fold at dinucleotide binding sites

The Rossmann fold is one of the most common and widely distributed super-secondary structures. It is composed of a series of alternating beta strand (β) and alpha helical (α) segments wherein the β-strands are hydrogen bonded forming a β-sheet. The initial beta-alpha-beta (βαβ) fold is the most conserved segment of Rossmann folds. As this segment is in contact with the ADP portion of dinucleotides such as FAD, NAD, and NADP it is also called as an "ADP-binding βαβ fold". The Proteopedia entry on the Rossmann fold (Available at: http://proteopedia.org/w/Rossmann_fold) was generated to illustrate several structural aspects of super families of FAD and NAD(P) binding proteins: (1) The coenzymes FAD and NAD(P) share the basic adenosine diphosphate (ADP) structure. (2) The βαβ fold motif that is common to both FAD and NAD(P) binding enzymes accommodates the common ADP component of these two coenzymes. (3) In both FAD and NAD(P) binding sites, the tight turn between the first β-strand and the α-helix is in contact with the two phosphate groups of ADP. (4) This hairpin curve includes the first two conserved glycines (Gly-x-Gly) that allow the sharp turn of the polypeptide backbone. (5) The two β-strands of the βαβ fold may constitute the core of a larger β-sheet that may include up to seven β-strands generally in parallel orientation. (6) The structures of segments between additional strands vary greatly and may be composed of a variety of structures such as multiple short helices or coils.

The evolution and complexity of the genes encoding the cytoskeletal proteins of human epidermal cells

In order to explore the differential expression and evolutionary conservation of the genes encoding the cytoskeletal proteins for human epidermal cells, we have constructed a library of bacterial plasmids containing inserts of double-stranded cDNAs complementary to the mRNAs of cultured human epidermal cells. Cloned hybrid plasmids containing 45-95% of the sequences present in keratin, actin, and tubulin mRNAs were isolated and characterized. To identify the cDNAs encoding the four major keratins of human epidermal cells, the clones were initially screened for their ability to hybridize strongly with 32P-labeled cDNA prepared from unfractionated epidermal mRNA (about 30% keratin mRNA). Strongly hybridizing clones were further characterized by positive hybrid selection. Two distinct classes of clones were identified: One class hybridized specifically to the 56 and 58kd keratin mRNAs and one class hybridized specifically to the 46 and 50kd keratin mRNAs. Each class is encoded by a separate multigene family of about 10 genes. The two classes of sequences are conserved throughout vertebrate evolution, indicating the functional importance of each class in filament assembly. Clones containing human cDNA sequences encoding a cytoplasmic actin and alpha-tubulin were selected by hybridization screening using 32P-labeled cloned cDNAs for the mRNAs of beta-actin and alpha-tubulin of embryonic chick brain. The identity of these clones was established by positive hybrid selection and by DNA sequence analysis. Similar to the keratins, the actins and tubulins are also encoded by multigene families which are highly evolutionarily conserved. The availability of cloned cDNAs specific for each of three types of epithelial cytoskeletal proteins allows us to investigate the coordinate expression of their mRNAs during terminal differentiation in human epidermis.

Conserved charged residues at the surface and interface of epithelial sodium channel subunits--roles in cell surface expression and the sodium self-inhibition response

The epithelial sodium channel (ENaC) is composed of three homologous subunits that form a triangular pyramid-shaped funnel, anchored in the membrane with a stem of six transmembrane domains. We examined the structure-function relationships of 17 conserved charged residues on the surface of the ectodomain of human γ-ENaC subunit by alanine mutagenesis and co-expression with α- and β-ENaC subunits in Xenopus oocytes. The results showed that Na(+) conductance of cells expressing these mutants can be accounted for by two parameters: (a) the ENaC density on the cell surface as measured by the fluorescence of an α-EnaC-yellow fluorescent protein hybrid and (b) the sodium self-inhibition (SSI) response that reflects the open probability of the channel (Po). Overall, the activity of all 17 mutants was correlated with surface levels of ENaC. There was no significant correlation between these parameters measured for α- and γ-ENaC subunit mutants at nine homologous positions. Thus, the functions of most of the homologous surface residues examined differ between the two subunits. Only four mutants (K328, D510, R514 and E518) significantly reduced the SSI response. The α-ENaC homologs of three of these (R350, E530 and E538) also severely affected the SSI response. The cASIC1 homologs of these (K247, E417, Q421) are located at the interface between subunits, on or about the ion pathway at the rotational symmetry axis in the center of the trimer. Thus, it is likely that these residues are involved in conformational changes that lead to channel constriction and the SSI response upon Na(+) ion flooding.

Epithelial sodium channels (ENaC) are uniformly distributed on motile cilia in the oviduct and the respiratory airways

Epithelial sodium channels (ENaCs) are located on the apical surface of cells and funnel Na(+) ions from the lumen into the cell. ENaC function also regulates extracellular fluid volume as water flows across membranes accompanying Na(+) ions to maintain osmolarity. To examine the sites of expression and intracellular localization of ENaC, we generated polyclonal antibodies against the extracellular domain of human α-ENaC subunit that we expressed in E. coli. Three-dimensional (3D) confocal microscopy of immunofluorescence using these antibodies for the first time revealed that ENaCs are uniformly distributed on the ciliary surface in all epithelial cells with motile cilia lining the bronchus in human lung and female reproductive tract, all along the fimbrial end of the fallopian tube, the ampulla and rare cells in the uterine glands. Quantitative analysis indicated that cilia increase cell surface area >70-fold and the amount of ENaC on cilia is >1,000-fold higher than on non-ciliated cell surface. These findings indicate that ENaC functions as a regulator of the osmolarity of the periciliary fluid bathing the cilia. In contrast to ENaC, cystic fibrosis transmembrane conductance regulator (CFTR) that channels chloride ions from the cytoplasm to the lumen is located mainly on the apical side, but not on cilia. The cilial localization of ENaC requires reevaluation of the mechanisms of action of CFTR and other modulators of ENaC function. ENaC on motile cilia should be essential for diverse functions of motile cilia, such as germ cell transport, fertilization, implantation, clearance of respiratory airways and cell migration.

Identification of the roles of conserved charged residues in the extracellular domain of an epithelial sodium channel (ENaC) subunit by alanine mutagenesis

Epithelial sodium channels (ENaC) are composed of three homologous subunits whose extracellular domains (ECD) form a funnel that directs ions from the lumen into the pore of ENaC. To examine the roles of conserved charged residues (Asp, Glu, Arg, and Lys) on ECD, we mutated 16 residues in human α-ENaC to alanine. The modified cRNAs were expressed in Xenopus laevis oocytes together with wild-type β- and γ-ENaC. The effect of each mutation was examined on three parameters: amiloride-sensitive Na(+) conductance (assayed by the two-electrode voltage-clamp method), Na(+)-dependent self-inhibition of ENaC, and oocyte cell surface expression of ENaC (quantitated by confocal microscopy of yellow fluorescent protein linked to γ-ENaC). Mutation of 13 of 16 residues reduced the ENaC Na(+) conductance (to 40-80% of WT). Mutation of only six residues showed a significant effect on the Na(+) self-inhibition time constant (τ). All 16 mutants showed a strong correlation between ENaC activity and oocyte surface expression (r = 0.62). Exclusion of four mutants showing the greatest effect on self-inhibition kinetics (Glu250 and Arg350 with τ = ~30% of WT, and Asp393 and Glu530 with τ = ~170% of WT) increased the correlation to r = 0.87. In the ASIC1 homotrimeric model, the homologs of α-ENaC Asp400 and Asp446 are exposed on the protein surface far from the other two chains. The mutations of these two residues showed the strongest effect on cell surface expression but had no effect on self-inhibition. Control mutations to a homologous charged residue (e.g., Asp to Glu) did not significantly affect ENaC activity. Changes in the two parameters, Na(+) self-inhibition and oocyte surface expression level, accounted for the magnitude of reduction in ENaC activity as a result of the mutation to Ala. These results establish that while some conserved charged residues are part of the structure responsible for Na(+) self-inhibition, most are essential for transport to the oocyte cell surface.

Autosomal recessive hyponatremia due to isolated salt wasting in sweat associated with a mutation in the active site of Carbonic Anhydrase 12

Genetic disorders of excessive salt loss from sweat glands have been observed in pseudohypoaldosteronism type I (PHA) and cystic fibrosis that result from mutations in genes encoding epithelial Na+ channel (ENaC) subunits and the transmembrane conductance regulator (CFTR), respectively. We identified a novel autosomal recessive form of isolated salt wasting in sweat, which leads to severe infantile hyponatremic dehydration. Three affected individuals from a small Bedouin clan presented with failure to thrive, hyponatremic dehydration and hyperkalemia with isolated sweat salt wasting. Using positional cloning, we identified the association of a Glu143Lys mutation in carbonic anhydrase 12 (CA12) with the disease. Carbonic anhydrase is a zinc metalloenzyme that catalyzes the reversible hydration of carbon dioxide to form a bicarbonate anion and a proton. Glu143 in CA12 is essential for zinc coordination in this metalloenzyme and lowering of the protein-metal affinity reduces its catalytic activity. This is the first presentation of an isolated loss of salt from sweat gland mimicking PHA, associated with a mutation in the CA12 gene not previously implicated in human disorders. Our data demonstrate the importance of bicarbonate anion and proton production on salt concentration in sweat and its significance for sodium homeostasis.

Truncated beta epithelial sodium channel (ENaC) subunits responsible for multi-system pseudohypoaldosteronism support partial activity of ENaC

Aldosterone regulated epithelial sodium channels (ENaC) are constructed of three homologous subunits. Mutations in the alpha-, beta- and gamma-ENaC subunit genes (SCNN1A, SCNN1B and SCNN1G) are associated with multi-system pseudohypoaldosteronism (PHA), and mutations in the PY motif of carboxy-terminal region of beta and gamma subunits are associated with Liddle syndrome of hereditary hypertension. In this study we identified two frameshift mutations in the SCNN1B alleles of a female infant diagnosed with multi-system PHA inherited from her parents. This is the first case of PHA in an Ashkenazi family in Israel. The p.Glu217fs (c.648dupA in exon 4) and p.Tyr306fs (c.915delC in exon 6) mutations produce shortened beta-ENaC subunits with 253 and 317 residues respectively instead of the 640 residues present in beta-ENaC subunit. Expression of cRNAs carrying these mutations in Xenopus oocytes showed that the mutations drastically reduce but do not eliminate ENaC activity. The findings reveal that truncated beta-ENaC subunits are capable of partially supporting intracellular transport of the other two subunits to the membrane and the final assembly of a weakly active channel together with normal alpha- and gamma-ENaC subunits. Moreover, these results enhance our understanding of the long-term consequences of these types of mutations in PHA patients.

Simple and efficient site-directed mutagenesis using two single-primer reactions in parallel to generate mutants for protein structure-function studies

BACKGROUND

In protein engineering, site-directed mutagenesis methods are used to generate DNA sequences with mutated codons, insertions or deletions. In a widely used method, mutations are generated by PCR using a pair of oligonucleotide primers designed with mismatching nucleotides at the center of the primers. In this method, primer-primer annealing may prevent cloning of mutant cDNAs. To circumvent this problem we developed an alternative procedure that does not use forward-reverse primer pair in the same reaction.

RESULTS

In initial studies we used a double-primer PCR mutagenesis protocol, but sequencing of products showed tandem repeats of primer in cloned DNA. We developed an alternative method that starts with two Single-Primer Reactions IN Parallel using high-fidelity Pwo DNA polymerase. Thus, we call the method with the acronym SPRINP. The SPRINP reactions are then combined, denatured at 95 degrees C, and slowly cooled, promoting random annealing of the parental DNA and the newly synthesized strands. The products are digested with DpnI that digests methylated parental strands, and then transformed into E. coli. Using this method we generated >40 mutants in cDNAs coding for human Epithelial Na+ Channel (ENaC) subunits. The method has been tested for 1-3 bp codon mutation and insertion of a 27 bp epitope tag into cDNAs.

CONCLUSION

The SPRINP mutagenesis protocol yields mutants reliably and with high fidelity. The use of a single primer in each amplification reaction increases the probability of success of primers relative to previous methods employing a forward and reverse primer pair in the same reaction.

Antioxidant Protective Mechanisms against Reactive Oxygen Species (ROS) Generated by Mitochondrial P450 Systems in Steroidogenic Cells

Mitochondrial P450 type enzymes catalyze central steps in steroid biosynthesis, including cholesterol conversion to pregnenolone, 11beta and 18 hydroxylation in glucocorticoid and mineralocorticoid synthesis, C-27 hydroxylation of bile acids, and 1alpha and 24 hydroxylation of 25-OH-vitamin D. These monooxygenase reactions depend on electron transfer from NADPH via FAD adrenodoxin reductase and 2Fe-2S adrenodoxin. These systems can function as a futile NADPH oxidase, oxidizing NADPH in absence of substrate, and leak electrons via adrenodoxin and P450 to O(2), producing superoxide and other reactive oxygen species (ROS). The degree of uncoupling depends on the P450 and steroid substrate. Studies with purified proteins and overexpression in cultured cells show consistently that adrenodoxin, but not reductase, is responsible for ROS production that can lead to apoptosis. In the ovary and corpus luteum, antioxidant enzyme activities superoxide dismutase, catalase, and glutathione peroxidase parallel steroidogenesis. Antioxidant beta-carotene, alpha-tocopherol, and ascorbate can protect against oxidative damages of P450 systems. In testis Leydig cells, steroidogenesis is associated with aging of the steroidogenic capacity.

Renin-aldosterone response, urinary Na/K ratio and growth in pseudohypoaldosteronism patients with mutations in epithelial sodium channel (ENaC) subunit genes

Multi-system pseudohypoaldosteronism (PHA) is a rare syndrome of aldosterone unresponsiveness characterized by symptoms of severe salt-losing caused by mutations in one of the genes that encode alpha, beta or gamma subunit of epithelial sodium channels (ENaC). We examined long-term changes in the renin-aldosterone response in patients with different mutations. Four PHA patients were followed-up for 7-22 years. Patient A with a heterozygous Gly327Cys missense mutation in alphaENaC is a mild case and patients B, C and D are severe cases. Two additional patients with renal PHA served as controls. In patient A, serum aldosterone and plasma renin activity (PRA) decreased with age, PRA reaching near normal values at age 11. In contrast, patients B-D showed a positive correlation between age and aldosterone (r>0.86 for all). In patient B with Arg508 stop mutation, aldosterone reached 166 nmol/L at age 19 (>300-fold higher than normal). Urinary Na/K ratios normalized gradually with age in all patients. Growth curves of the patients were reflective of the severity of PHA and compliance with salt therapy. Functional expression studies in oocytes showed that ENaC with alphaGly327Cys mutation, as observed in patient A, showed nearly 40% activity of the wild type ENaC. In contrast, stop mutation as in patient B reduces ENaC activity to less than 5% of the normal. Our results demonstrate distinct genotype-phenotype relationships in multi-system PHA patients. The degree of ENaC function impairment affects differently the renin-aldosterone system and urinary Na/K ratios. The differences observed are age-dependent and PHA form specific.

Novel mutations in epithelial sodium channel (ENaC) subunit genes and phenotypic expression of multisystem pseudohypoaldosteronism

OBJECTIVES

Multisystem pseudohypoaldosteronism (PHA) is a rare autosomal recessive aldosterone unresponsiveness syndrome that results from mutations in the genes encoding epithelial sodium channel (ENaC) subunits alpha, beta and gamma. In this study we examined three PHA patients to identify mutations responsible for PHA with different clinical presentations.

PATIENTS

All three patients presented uniformly with symptoms of severe salt-loss during the first week of life and were hospitalized for up to a year. Beyond infancy, one of the patients showed mild renal salt loss and had no lower respiratory tract infections until 8 years of age, while the other patients continue with a severe course.

RESULTS

We sequenced the complete coding regions and intron-exon junctions of the genes encoding alpha, beta and gamma subunits of ENaC for all patients. The results revealed that the mild case represents a novel compound heterozygote including a missense (Gly327Cys) mutation in the alphaENaC gene. Sequences of relatives over three generations confirmed that the missense mutation co-segregates with PHA. This mutation was not found in 60 control subjects. The other patients with severe PHA had two homozygous mutations, a novel deletion mutation in exon 8 of the alphaENaC gene and a splice site mutation in intron 12 of the betaENaC gene. Most of the PHA-causing mutations appear in the alphaENaC gene located on chromosome 12 rather than in the beta and gammaENaC genes located tandemly on chromosome 16. However, the frequency of sequence variants in patients and control subjects showed no difference between genes.

CONCLUSIONS

Severe PHA cases are associated with mutations leading to absence of normal-length alpha, beta or gammaENaC, while a mild case has been found to be associated with a missense mutation in alphaENaC. The predominance of PHA-causing mutations in the alphaENaC gene may be related to the function of this subunit.

ACTH induces TIMP-1 expression and inhibits collagenase in adrenal cortex cells

To identify genes that are induced by corticotropin (ACTH) in adrenal cortex cells, we carried out a differential hybridization screening of adrenal cortex cDNA libraries. Some of the clones we identified represented tissue inhibitor of metalloproteinase 1 (TIMP-1) mRNA. We examined ACTH dependence of the expression of TIMP-1 in vitro in cultured bovine adrenocortical cells, and in ACTH-treated rats. Northern blot analysis of total RNA from cells showed that the level of TIMP-1 mRNA increases sharply within 3h after ACTH stimulation. Since TIMP-1 inhibits some cell matrix metalloproteinases (MMPs) of the collagenase type, we examined the effect of ACTH on collagenase activity in bovine adrenocortical cells. Exposure of confluent cultures to ACTH for 24h showed dose-dependent inhibition of collagenase activity. Northern blot analysis of total RNA from rat adrenal zona fasciculata-reticularis and zona glomerulosa showed that in both of these zones TIMP-1 expression was induced within 12h after ACTH injection. Long-term (9 days) treatment with ACTH increased TIMP-1 mRNA levels nearly sixfold in zona fasciculata-reticularis. Overall, our results show that ACTH causes induction of TIMP-1 and suppression of collagenase activity, and suggest that ACTH may modulate the activities of MMPs and hence cell matrix remodeling.

Novel mutations responsible for autosomal recessive multisystem pseudohypoaldosteronism and sequence variants in epithelial sodium channel alpha-, beta-, and gamma-subunit genes

Multisystem pseudohypoaldosteronism (PHA), is a syndrome of unresponsiveness to aldosterone with autosomal recessive inheritance. Previously we showed that mutations in the epithelial sodium channel (ENaC) alpha-, beta-, and gamma-subunits are responsible for PHA. In this study we examined four independent probands with multisystem PHA, three of whom were born to consanguineous parents. In our search for mutations we also determined the complete coding sequences of each of the three genes encoding alpha-, beta-, and gamma-subunits in individuals representing different ethnic groups. Our analyses revealed the following homozygous mutations in three probands: 1) insertion of a T in exon 8 of the alpha ENaC gene that causes a frameshift error at Tyr(447) and leads to a premature stop codon at K459 in a Pakistani patient; 2) R508stop mutation in exon 11 of the alpha ENaC gene in an Indian patient; and 3) a splice site mutation in intron 12 of the beta ENaC gene (1669 + 1 g-->a) in a Scottish patient. The parents were heterozygous for the latter two mutations. The second mutation was previously observed in an Iranian Jewish patient. Our sequencing of the alpha-, beta-, and gamma-coding sequences revealed some sequence variants, some of which may represent single nucleotide polymorphisms. The gamma-subunit protein sequence was completely conserved in the six subjects examined. The homozygous mutations identified in the alpha and beta ENaC genes should result in reduced or abolished ENaC activity in PHA patients, explaining the disease symptoms.

Amiloride-sensitive epithelial sodium channel subunits are expressed in human and mussel immunocytes

In this study, we examined the expression of epithelial Na(+) channel (ENaC) subunits in human peripheral blood lymphocytes, human lymph nodes and molluscan immunocytes using non-radioactive in situ hybridization. The results showed that T lymphocytes express the ENaC gamma subunit mRNA, and B lymphocytes the ENaC beta subunit mRNA. Yet, the alpha subunit mRNA was not detected in either cell type. In molluscan immunocytes, all three homologous ENaC subunit mRNAs are present, and these data were also confirmed by RT-PCR and sequencing of the PCR products. These findings show evolutionary conservation of the expression of ENaC subunits in immunocytes of invertebrates to vertebrates. The observed differential expression patterns of ENaC subunits suggest that ENaC function may be regulated differentially in different types of human lymphocytes.

The structure of adrenodoxin reductase of mitochondrial P450 systems: electron transfer for steroid biosynthesis

Adrenodoxin reductase is a monomeric 51 kDa flavoenzyme that is involved in the biosynthesis of all steroid hormones. The structure of the native bovine enzyme was determined at 2.8 A resolution, and the structure of the respective recombinant enzyme at 1.7 A resolution. Adrenodoxin reductase receives a two-electron package from NADPH and converts it to two single electrons that are transferred via adrenodoxin to all mitochondrial cytochromes P 450. The structure suggests how the observed flavin semiquinone is stabilized. A striking feature is the asymmetric charge distribution, which most likely controls the approach of the electron carrier adrenodoxin. A model for the interaction is proposed. Adrenodoxin reductase shows clear sequence homology to half a dozen proteins identified in genome analysis projects, but neither sequence nor structural homology to established, functionally related electron transferases. Yet, the structure revealed a relationship to the disulfide oxidoreductases, permitting the assignment of the NADP-binding site.

Chaperone-assisted expression of authentic bovine adrenodoxin reductase in Escherichia coli

Adrenodoxin reductase is an essential component of the mitochondrial monooxygenase systems that are involved in the synthesis of steroid hormones and related compounds. After removing by mutagenesis a secondary ribosome binding site and an mRNA loop formed between the gene and the vector, large amounts of the enzyme could be produced in Escherichia coli by coexpression with the HSP60-chaperone system. The purified protein was homogeneous enough for reproducible crystallization. The crystals diffracted X-rays isotropically beyond 1.7 A resolution permitting a structure analysis.

Gene structure of the human amiloride-sensitive epithelial sodium channel beta subunit

ENaC functions in the transport of sodium ions across epithelial cells and consequently regulates blood volume and pressure. ENaC complex includes at least three different subunits, alpha, beta, and gamma, which are developmentally regulated and differentially controlled by aldosterone. In this study, we determined the exon-intron organization of the beta ENaC subunit by sequencing genomic DNA from three subjects from three different ethnic groups. The results showed that the coding region of the human betaENaC gene (SCNN1B) extends from exon 2 to exon 13. No polymorphism was observed in the examined subjects, indicating strict conservation of the coding region sequence. The introns of beta subunit gene are located at exactly the same positions as in the alpha and gamma subunits, although these proteins share only 26-32% sequence identity. These results thus elucidate the gene structure of the beta subunit and indicate that exon-intron architecture of the three genes encoding the three subunits of ENaC have remained highly conserved despite the divergence of their sequences.

Antioxidant capacity is correlated with steroidogenic status of the corpus luteum during the bovine estrous cycle

The reactions of steroid hormone biosynthesis are accompanied by formation of oxygen radicals. We determined the levels of some antioxidants and antioxidative enzymes at different developmental stages of bovine corpora lutea to examine their correlation with steroidogenic status. Plasma progesterone concentrations of estrous cycle synchronized cows increased until day 16, and then decreased rapidly during luteal regression. The levels of steroidogenic cytochrome P450scc and adrenodoxin paralleled the changes in plasma progesterone. Among the antioxidative enzymes examined, the SOD and catalase activities showed patterns most similar to plasma progesterone. Catalase and SOD activities increased 6-8 fold from day 6 to 16 of the estrous cycle and then decreased during the luteal regression. Ascorbate and beta-carotene showed low but significant correlation with P450scc and plasma progesterone levels. The profiles of two lipophilic antioxidants in corpora lutea were very different. beta-carotene concentration increased by approximately 6 fold from day 6 to 16, and decreased in regressive tissue. alpha-tocopherol showed a 3 fold increase between days 6 and 9 followed by a rapid decrease. Thus, at the peak of steroidogenesis at mid-luteal phase alpha-tocopherol levels decreased, but beta-carotene levels increased. The correlation between the levels of some antioxidant enzymes and compounds with progesterone levels indicates that antioxidative mechanisms are activated to cope with steroidogenesis dependent oxyradical formation in the bovine corpus luteum.

In situ localization of ACTH receptor-like mRNA in molluscan and human immunocytes

Adrenocorticotropin hormone (ACTH) receptor-like messenger RNA was localized in molluscan hemocytes and human peripheral blood mononuclear cells by in situ hybridization using a digoxigenin-labelled bovine complementary DNA probe. These findings suggest that the ACTH receptor gene has been highly conserved during evolution. Moreover, these data represent further support for a relationship between the immune and neuroendocrine systems in invertebrates, as documented in our previous studies.

Pseudohypoaldosteronism due to renal and multisystem resistance to mineralocorticoids respond differently to carbenoxolone

Type I pseudohypoaldosteronism (PHA) is a hereditary syndrome of salt wasting resulting from unresponsiveness to mineralocorticoids. PHA is manifested in two clinically and genetically distinct forms, affecting either only the kidney or multiple target organs of aldosterone. We examined the mineralocorticoid effect of carbenoxolone (CBX) in young PHA patients with either renal or multisystem resistance to aldosterone to find out whether CBX may help reduce the requirement for a high-salt diet. CBX did not show any significant salt-retaining effect in two patients with multiple PHA, and did not affect the renin-aldosterone system. In contrast, CBX significantly suppressed the renin-aldosterone system in a renal PHA patient for the whole duration of treatment, but without a long-term salt-retaining effect. On CBX treatment, urinary cortisone levels decreased and the cortisol:cortisone ratio increased, indicating that CBX inhibited 11beta-HSD activity that metabolizes cortisol to cortisone. The complete lack of effect of CBX on the renin-aldosterone system in multisystem PHA patients indicates that CBX does not exert an effect via mineralocorticoid (MR) or glucocorticoid receptors. Examination of the structure and expression of the MR gene by Southern blot analysis and polymerase chain reaction (PCR) showed no abnormality. Whereas multiple PHA results from a spectrum of mutations in the mineralocorticoid activated epithelial sodium channel subunits, the genetic basis of renal PHA is still unknown. The response to CBX suggests that there is at least a partly functional MR in renal PHA patients.

Mutations in subunits of the epithelial sodium channel cause salt wasting with hyperkalaemic acidosis, pseudohypoaldosteronism type 1

Autosomal recessive pseudohypoaldosteronism type I is a rare life-threatening disease characterized by severe neonatal salt wasting, hyperkalaemia, metabolic acidosis, and unresponsiveness to mineralocorticoid hormones. Investigation of affected offspring of consanguineous union reveals mutations in either the alpha or beta subunits of the amiloride-sensitive epithelial sodium channel in five kindreds. These mutations are homozygous in affected subjects, co-segregate with the disease, and introduce frameshift, premature termination or missense mutations that result in loss of channel activity. These findings demonstrate the molecular basis and explain the pathophysiology of this disease.

Localisation of pseudohypoaldosteronism genes to chromosome 16p12.2-13.11 and 12p13.1-pter by homozygosity mapping

Pseudohypoaldosteronism type 1 (PHA1, OMIM 264350) is a rare Mendelian disorder characterised by end-organ unresponsiveness to mineralocorticoids. Most steroid hormone insensitivity syndromes arise from mutations in the corresponding receptor, but available genetic evidence is against involvement of the mineralocorticoid receptor gene, MLR, in PHA1. A complete genome scan for PHA1 genes was undertaken using homozygosity mapping in 11 consanguineous families. Conclusive evidence of linkage with heterogeneity was obtained with a maximum two-locus admixture lod score of 9.9. The disease locus mapped to chromosome 16p12.2-13.11 in six families and to 12p13.1-pter in the other five families. The two chromosomal regions harbour genes for subunits of the amiloride-sensitive epithelial sodium channel: SCNN1B and SCNN1G on 16p and SCNN1A on 12p. Liddle's syndrome of hypertension and pseudoaldosteronism has been shown to arise from mutations in SCNN1B and SCNN1G. These results strongly suggest that PHA1 and Liddle's syndrome are allelic variants caused by mutations in genes encoding subunits of this sodium channel. These genes are of broad biological interest both in relation to sodium and water homeostasis in mammals and by virtue of their homology to the mec genes of Caenorhabditis elegans involved in mechanosensitivity and neuronal degeneration.

Tissue- and species-dependent expression of sheep lung microsomal cytochrome P4502B(LgM2)

Expression of sheep lung microsomal cytochrome P4502B(LgM2) isozyme was determined in lung and liver preparations from rabbits, rats, sheep, cows and humans. Several tissues of sheep such as lung, liver, adrenal, kidney, ovary, intestine, muscle, spleen, pancreas and brain were also examined. Tissue homogenates were analyzed by Western-blotting with an antibody raised against purified sheep lung P4502B(LgM2) isozyme in rabbits. This isozyme was detected in lung samples from every species examined. Homologues of cytochrome P4502B(LgM2) were expressed in livers of sheep and rabbits but not in those of humans and cows. Although homologous forms of P4502B(LgM2) were not expressed in sheep brain, ovary, pancreas, spleen and muscle tissues, they were expressed in sheep adrenal, intestine and kidney.

Exclusion of the locus for autosomal recessive pseudohypoaldosteronism type 1 from the mineralocorticoid receptor gene region on human chromosome 4q by linkage analysis

Pseudohypoaldosteronism type 1 (PHA1) is an uncommon inherited disorder characterized by salt-wasting in infancy arising from target organ unresponsiveness to mineralocorticoids. Clinical expression of the disease varies from severely affected infants who may die to apparently asymptomatic individuals. Inheritance is Mendelian and may be either autosomal dominant or autosomal recessive. A defect in the mineralocorticoid receptor has been implicated as a likely cause of PHA1. The gene for human mineralocorticoid receptor (MLR) has been cloned and physically mapped to human chromosome 4q31.1-31.2. The etiological role of MLR in autosomal recessive PHA1 was investigated by performing linkage analysis between PHA1 and three simple sequence length polymorphisms (D4S192, D4S1548, and D4S413) on chromosome 4q in 10 consanguineous families. Linkage analysis was carried out assuming autosomal recessive inheritance with full penetrance and zero phenocopy rate using the MLINK program for two-point analysis and the HOMOZ program for multipoint analysis. Lod scores of less than -2 were obtained over the whole region from D4S192 to D4S413 encompassing MLR. This provdes evidence against MLR as the site of mutations causing PHA1 in the majority of autosomal recessive families.

Selective increases in adrenal steroidogenic capacity during acute respiratory disease in infants

To examine steroidogenic responses of the different zones of the adrenal cortex to acute disease we determined the basal and adrenocorticotropin (ACTH)-stimulated levels of cortisol, dehydroepiandrosterone (DHEAS) and aldosterone in 16 infants aged 1-4 months with acute bronchiolitis. Fourteen of the infants were retested after recovery. During illness the mean basal levels of cortisol and DHEAS were twice as high as the levels after recovery (370 vs 180 nmol/l and 2.7 vs 1.3 mumol/l, respectively). The mean peak ACTH-stimulated levels of cortisol and DHEAS during illness were 1.5- and 2.5-fold higher, respectively, than the levels found after recovery. Although aldosterone secretion was stimulated > or = 3-fold by ACTH, illness was not associated with any change in aldosterone secretory capacity. The basal and stimulated levels of both cortisol and DHEAS during illness and after recovery were correlated significantly. Thus, the relative steroidogenic capacities for these two steroids were characteristic of the individual infant and showed constancy over a period of at least several weeks. While the levels of cortisol and aldosterone were not dependent on the age of the infants, both the basal and stimulated levels of DHEAS correlated strongly with age. We conclude that during acute disease the steroidogenic capacity selectively increases in the zones that secrete cortisol and DHEAS (only in infants < 3 months) but not in the zona glomerulosa that secretes aldosterone. The DHEAS response may be related to its putative effects to enhance immune responses.

Electron leakage from the adrenal cortex mitochondrial P450scc and P450c11 systems: NADPH and steroid dependence

In the present study we examined the coupling of NADPH oxidation to substrate hydroxylation and the effects of steroids on this process in reconstituted P450scc and P450c11 systems. To determine the relative rates of substrate hydroxylation vs electron leakage we assayed both the steroid product and H2O2 in the same sample. For both P450 systems the rates of steroid product and superoxide formation increased as NADPH concentration was increased. However, P450c11 was found to be more leaky. The leakage from the P450scc system was not affected by pregnenolone, the product of cholesterol side chain cleavage. In contrast, corticosterone, the product of P450c11, increased the rate of futile NADPH oxidation by the P450c11 system. We also tested a series of steroids to analyze the stereospecificity of their effects. Relative to the control without steroid, both C-19 and C-21 steroids with 11 alpha-hydroxy groups (11 alpha-OH-testosterone and 11 alpha-OH-cortisol) decreased leakage, and those with 11 beta-OH groups (11 beta-OH-testosterone and cortisol) stimulated both NADPH oxidation and electron leakage as measured by H2O2 formation. The results revealed a correlation between the effects previously observed in living cells and in our reconstituted systems. These findings provide further evidence that mitochondrial P450 systems indeed function as a significant source of oxygen radicals in steroidogenic cells.

Routes and regulation of NADPH production in steroidogenic mitochondria

The first and rate-limiting step of steroidogenesis is catalyzed by the mitochondrial cholesterol side chain cleavage system that is dependent on NADPH. The pathways of NADPH generation in steroidogenic mitochondria include three major routes catalyzed by: 1. NADP-linked malic enzyme, 2. NADP-linked isocitrate dehydrogenase, and 3. nicotinamide nucleotide transhydrogenase. The main route may differ among cell types and across species. Generally operation of alternative routes, with different substrates is not excluded. The oxidation of NADPH by the mitochondrial P450 systems is not tightly coupled with substrate metabolism, as these systems can reduce O2 by a single electron to produce harmful superoxide radical. To minimize such futile NADPH oxidation, NADPH generation may be regulated by two types of mechanisms: 1. Feedback mechanisms that maintain the ratio of NADPH/NADP+ at a steady-state level by enhancing the rate of NADPH production to keep up with its rate of oxidation, e.g., allosteric regulation of enzymes involved in NADPH production. 2. Hormonal signals that enhance the level of NADPH production in coordination with steroidogenesis. One major hypothesis with experimental evidence is that stimulation of mitochondrial NAD(P)H synthesis is mediated by Ca++ as a second messenger of tropic factors. Tropic stimulation of cells increases the levels of Ca++ in the cytosol and then in the mitochondrial matrix, and the rise in Ca++ activates enzymes involved in NAD(P)H synthesis. These regulatory mechanisms most probably operate in concert adjusted to the steroidogenic activity of the cell.

Cloning of ACTH-regulated genes in the adrenal cortex

To search for genes that are induced by ACTH in adrenocortical cells, we screened adrenal cortex cDNA libraries by a differential hybridization method using cDNA probes representing mRNAs from cells with or without ACTH stimulation. Forty clones were identified as ACTH induced (yielding a frequency of about 1/2500 plaques screened), and two clones as ACTH repressed. The cDNAs isolated and sequenced include nuclear genes for microsomal steroidogenic enzymes and novel proteins of yet unidentified functions, and mitochondrial genes encoding subunits of oxidative phosphorylation enzymes. Northern blot analysis of RNA from cells stimulated with ACTH confirmed the induction of these genes by ACTH, yet revealed important differences in the relative responses of the respective mRNAs. The time courses showed the major increase in the initial 6 h; and a decline after 24-36 h. The enhancement of the levels of the mRNAs could be ascribed to transcriptional activation. Since the mitochondrial genome is transcribed as a single polycistronic unit, to account for the > 20-fold differences in the levels of the mitochondrial mRNAs it is necessary to invoke differential stabilities of these mRNAs. The synchronous increase in the expression of both the steroidogenic enzymes and the mitochondrial oxidative phosphorylation system subunits, provides evidence for coregulation of steroidogenic and energy producing capacities of adrenal cells to meet the metabolic needs of steroid hormone production. Suppression of beta-actin gene expression may be related to changes in actin polymerization during ACTH-dependent cytoskeletal reorganization.

A fluorimetric assay for hydrogen peroxide, suitable for NAD(P)H-dependent superoxide generating redox systems

We report a simple and sensitive fluorimetric method for quantitative assay of the production rate of hydrogen peroxide, and indirectly of superoxide, during electron transfer reactions. The assay requires the inclusion of superoxide dismutase, catalase, and 6% methanol in the tested reaction system, to stochiometrically produce formaldehyde per molecule of H2O2 generated. The reaction is terminated by adding 2 vol of Nash reagent and heating at 60 degrees C for 10 min, to convert accumulated formaldehyde to diacetyldihydrolutidine (DDL). The standard curve for formaldehyde, based on the fluorescence of DDL, is highly reproducible and allows measurement of 1 microM amounts in the reaction sample (coefficient of variation < 15%). The excitation and emission wavelengths of DDL at 412 and 505 nm are distant from those of NAD(P)H. Thus, the method can be used in NAD(P)H-dependent enzymatic systems to measure both NAD(P)H oxidation and superoxide production in the same sample. We validated the assay in a mitochondrial P450 system determining the fraction of total electron flow that is channeled to oxy-radical formation. The assay should be useful in the study of this and other superoxide/H2O2 generating systems.

cDNA cloning and sequence analysis of the bovine adrenocorticotropic hormone (ACTH) receptor

We isolated five independent cDNAs of nearly 3000 bp for the bovine ACTH receptor by screening adrenal cortex cDNA libraries with a PCR cloned cDNA fragment. The deduced receptor sequence includes 297 residues (M(r) = 33,258) with 81% identity with the human ACTH receptor, and shows seven hydrophobic transmembrane domains. The calculated M(r) of the receptor is smaller than the 40-45 kDa observed in crosslinking studies with labeled ACTH. Since the bovine and human receptors have two glycosylation motifs in the N-terminus, the difference may result from glycosylation of the receptor. Analysis of the sequences of both bovine and human receptors revealed a single protein kinase. A phosphorylation motif located in the third intracellular loop (Ser-209) juxtaposed to a protein kinase C phosphorylation motif (Thr-204). Thus, the involvement of protein kinase A and C pathways in ACTH action may be mediated in part by phosphorylation of the ACTH receptor at these motifs. The 3'-untranslated region of the bovine cDNA is > 2000 bp and includes two inverse repeats giving an extensive and strong secondary structure to the ACTH receptor RNA.

Cloning of LL5, a novel protein encoding cDNA from a rat pituitary library

While screening a rat pituitary cDNA library for a peptide hormone receptor, we identified a cDNA that represents a novel gene. The 3.8 kb cDNA has an open reading frame of 2.3 kb encoding a protein of 781 amino acids (M(r) = 87,507). Southern blot analysis of rat, mouse, bovine and human genomic DNAs revealed that a homologous gene is present in these species probably in a single copy. Northern blot analysis showed that in addition to the pituitary gland, the gene is also expressed in other rat tissues. Scanning of DNA and protein databanks revealed no significant homology to any other sequence. Thus, this gene encodes a heretofore unidentified protein.

Mitochondrial-genome-encoded RNAs: differential regulation by corticotropin in bovine adrenocortical cells

Differential screening of an adrenal cortex cDNA library for corticotropin (ACTH)-inducible genes led to the isolation of a group of cDNAs representing mitochondrial genes that encode subunits of cytochrome oxidase, ATPase, and NADH dehydrogenase. Northern blot analysis of RNA from cells stimulated by ACTH confirmed the induction of these genes by ACTH yet revealed major differences in the relative responses of the respective mRNAs. The levels of mRNAs for cytochrome oxidase subunit I and ATPase increased 2- to 4-fold and for NADH dehydrogenase subunit 3 increased 20-fold, whereas the levels of the mitochondrial 16S rRNA showed no change within 6 h of ACTH stimulation. These effects of ACTH on mitochondrial mRNA levels probably result from both activation of the H2 transcription unit that encodes mitochondrial mRNAs and alteration of mRNA stability. ACTH also increased the activity of cytochrome oxidase after 12 h of stimulation. Examination of the tissue specificity of expression of five mitochondrial genes showed a wide range of RNA levels among 11 tissues but high correlations between individual RNA levels, consistent with a coordinated expression of the mitochondrial genes, although at different levels in each cell type. Proportionately high levels of mitochondrial mRNAs were found in adrenal cortex, probably reflecting a stimulatory effect of ACTH in vivo. Overall, the results indicate that ACTH enhances the energy-producing capacity of adrenocortical cells.